This invention concerns improvements in or relating to lenses and relates more particularly to lenses, and to afocal telescopes, for use at infra-red wavelengths.
Infra-red lenses and afocal telescopes are known which operate in the 3 to 5 micron waveband, as are infra-red lenses and afocal telescopes which operate in the 8 to 12 micron waveband. The differences usually lie basically in the infra-red transmitting materials employed. Infra-red transmitting materials tend to have a high rate of change of refractive index with wavelength and hence give rise to serious problems of chromatic aberration. These problems are enhanced by increasing the required operative waveband and are therefore very considerable if one lens or telescope is required to operate over both the 3 to 5 and the 8 to 12 microns wavebands.